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TSM53: THE EXTERNAL, MIDDLE AND INNER EAR 28/10/08 LEARNING OUTCOMES Describe the structure of the external ear, the middle ear and the inner ear EXTERNAL EAR The external ear consists of the auricle and external auditory meatus The auricle or pinna is a mostly immobile cartilaginous ‘flap’ that aids sound localisation o Outer helix and inner antihelix – cartilaginous ridges o Laterally projecting crura of the antihelix superiorly and of the helix inferiorly o Anterior tragus and infero-posterior antitragus – cartilaginous prominences o Inferior lobule (‘ear lobe’) continuous with helix – soft fatty projection The external auditory meatus or canal is a narrow passage leading to the tympanum o Lateral third has cartilaginous walls, medial two thirds are osseous within the temporal bone MIDDLE EAR – INTERNAL STRUCTURE The middle ear consists of the tympanic cavity and ossicles The tympanic membrane is effectively the border between the external and middle ear o Lateral surface of the membrane faces infero-anteriorly o Superior prominence made by the lateral process of malleus o Internal folds on either side of the lateral process – anterior and posterior malleolar folds Superior to these folds the membrane is thin and non-fibrous – pars flaccida Inferior to the folds it is thick and tense – pars tensa o Internal inferior projection from the lateral process – handle of malleus Attaches centrally to the pars tensa at the umbo When using an otoscope to view a healthy tympanic membrane an antero-inferior light reflection can be seen – the cone of light – which is useful for orientation when viewing otoscopic images The three ossicles link together within the tympanic cavity to transmit sound to the inner ear o The malleus is ‘T’ shaped and articulates with the incus superiorly at its head o The incus is ‘L’ shaped, articulates with the malleus at its base and the stapes at its long limb o The stapes is ‘D’ shaped, articulates with the incus at its head and the oval window at its base MIDDLE EAR – WALLS There are four principal openings in the walls of the middle ear: o Labyrinthine (medial) wall – border of the inner ear Oval window – entrance to the vestibule, sealed by the footplate of stapes Round window – entrance to the scala tympani, sealed by mixed fibrous membrane o Anterior wall Eustachian tube – to the nasopharynx, facilitating pressure equalisation o Mastoid (posterior) wall Aditus to mastoid antrum – to the air cells of the mastoid process INNER EAR – OVERVIEW The inner ear consists of the vestibule, semicircular canals and cochlea within a bony labyrinth o This bony labyrinth is part of the petrous temporal bone o Unlike the external and middle ears, all parts of the inner ear are fluid filled The bony labyrinth itself is filled with clear perilymph Within the bony labyrinth is a membranous labyrinth filled with endolymph The membranous labyrinth comprises two ducts and two sacs: o Utricle and saccule – within the vestibule, part of the vestibular system o Semicircular ducts – within the semicircular canals, part of the vestibular system o Cochlear duct – the passage within the cochlear through which sound is transmitted Describe the muscles of the middle ear There are two muscles associated with the middle ear: o Tensor tympani – tenses the tympanum in response to loud sounds – reduces vibrations Runs above and alongside the auditory tube Inserts onto upper handle of malleus behind the tympanum Innervated by a branch of trigeminal V3 (mandibular nerve) o Stapedius – displaces the stapes posteriorly in response to loud sounds – reduces oscillation Arises from the pyramidal eminence in the mastoid (posterior) wall Inserts onto the neck of stapes Innervated by a branch of the facial nerve (CNVII) Describe the functions of the external and middle ears Sound striking the pinna is eventually recognised by the brain as external sound o Inner-ear headphones effectively bypass the pinna and are perceived as ‘internal sound’ o The external ear also aids sound localisation in the vertical plane The external auditory meatus protects components of the middle and inner ear from damage o Dust, foreign bodies and so on are captured in a waxy secretion – cerumen The middle ear acts as a transformer between the air-filled external ear and fluid-filled inner ear o Sound waves passing directly from air to fluid are mostly reflected o Since fluid is much denser than air it has a far higher acoustic impedance o Matching input impedance to output impedance minimises reflection Specific acoustic impedance (z) of a medium involving a single wave frequency is: o where p is sound pressure, v is particle velocity The middle ear concentrates sound pressure and reduces particle velocity thus increasing the ‘input impedance’ into the fluid-filled inner ear to minimise reflection o The surface area of the tympanum is roughly 18 times that of the footplate of stapes o The intermediate ossicles also act as levers which amplify the force by a factor of 1.3 o The ratio of force to surface area corresponding to pressure in total increases 25-fold Describe the role of the inner ear in sound transduction INNER EAR – COCHLEA The cochlea is a stacked bony spiral with two to three ‘levels’ containing the organs of hearing o The central bony column of the cochlea is called the modiolus o The cochlear nerve (branch of CNVIII) ascends the core of the modiolus o A bony shelf, the spiral lamina, projects laterally from the modiolus throughout the cochlea o The endolymph-filled cochlear duct encircles the modiolus and attaches to the spiral lamina o Above the cochlear duct runs another canal, the scala vestibuli, continuous with the vestibule o Below the cochlear duct runs the scala tympani, terminating at the round window o Point of connection between the scala vestibuli and tympani at the apex, the helicotrema The cochlear duct is triangular with its base facing laterally and apex medially towards the modiolus o Its outer wall (spiral ligament) has a thickened epithelial layer, the stria vascularis o Its roof has a thin membrane separating it from the scala vestibuli, Reissner’s membrane o Its floor has a thicker membrane separating it from the scala tympani, the basilar membrane The organ of hearing, the organ of Corti, is attached to the basilar membrane o Has an overlying tectorial membrane which interacts with stereocillia of underlying hair cells Three or four layers of outer hair cells connect directly to the membrane Single layer of inner hair cells does not connect directly o Cochlear nerve fibres are stimulated by neurotransmitter released by the inner hair cells only INNER EAR – SOUND TRANSDUCTION Pressure waves entering the inner ear via the oval window propagate through the vestibule o These waves ascend the scala vestibuli to the helicotrema at the apex of the cochlea o They then descend the scala tympani which terminates at the round window As sound waves ascend the scala vestibuli they cause the basilar membrane to vibrate and displace o Displacement of the basilar membrane excites the hair cells o High frequencies produce maximal displacement at the base of the cochlea o Low frequencies produce maximal displacement at the apex of the cochlea Distortion of the stereocillia of the hair cells moderates membrane potassium channels o Bending ‘outward’ triggers potassium influx and depolarisation o Bending ‘inward’ reduces potassium influx and promotes hyperpolarisation Depolarisation of the inner hair cells causes the release of an excitatory neurotransmitter o This stimulates nearby cochlear nerve fibres and transmits information to the CNS The outer hair cells are essentially highly sensitive modifiers and amplifiers o Enable our hearing to be as sensitive as it is o Damaged by some antibiotics, particularly aminoglycosides (broad spectrum)